1. Field of the Invention
The present invention relates generally to a source of hydraulic fluid under pressure and more specifically to a source of hydraulic fluid under pressure which is particularly suited for use with an active suspension for the control of vehicular roll and pitch.
2. Description of the Prior Art
JP-A-63-251313 discloses an arrangement wherein a variable displacement type pump arrangement has been operatively connected with a source or rotational energy such as a prime mover (e.g. internal. combustion engine) of an automotive vehicle, and the output of the pump supplied to an active vehicle suspension. In this arrangement vertical, lateral and fore/aft accelerations are used to determine the change in vehicle posture. When the change in posture exceeds a predetermined level, the amount of discharge is increased above a level used when the vehicle is stationary, and supplied (by way of example) to a hydraulic fluid cylinder operatively connected between the sprung and unsprung components of the vehicle suspension.
However, this arrangement has suffered from a problem in that even though it aims to take advantage of the fact that the amount of power which is consumed when the vehicle is stationary is less than that when the vehicle is running, the amount of pump discharge is always increased in response to large changes in vehicle posture.
As the time related variation in lateral acceleration at high vehicle speeds is higher than that at low speeds, it is necessary to take the lateral vehicle acceleration into account when estimating the amount of hydraulic fluid which is required during high speed modes of operation, the pump output is controlled on the high side during both high and low speed operations with the result that the pump load is undesirably increased and exerts a detrimental effect on the vehicle fuel economy.